More than one-third of children with Crohn's disease suffer from growth failure. Given the rising incidence and earlier diagnosis of Crohn's disease, nutrition and growth are critical outcomes for these children. There is a lack of knowledge concerning the response of these parameters to new therapies for Crohn's disease. Infliximab, an anti-tumor necrosis factor-alpha antibody, has revolutionized the care of children with Crohn's disease recalcitrant to corticosteroid therapy or complicated by fistulizing disease. We hypothesize that children with active Crohn's disease will have increased energy expenditure and proteolysis, and that treatment with infliximab will lead to greater improvement in these parameters than traditional corticosteroid therapy. To test these hypotheses, children with active relapsed Crohn's disease will be enrolled in a prospective study. Outpatient metabolic studies will be conducted just before, tow and fourteen weeks after initiation of corticosteroid of infliximab therapy. Using stable essential amino acid isotopes, rates of total body proteolysis and albumin synthesis can be measured in fasting, and enterally and parenterally fed states. Resting energy expenditure, using indirect calorimetry, and total energy expenditure, using doubly-labeled water, will be calculated at each visit. The proposed work is innovative, as it expands the knowledge of protein and energy use in response to traditional and novel therapies for pediatric Crohn's disease. These results will also help allow children with Crohn's disease to reach their true growth potential. The principal investigator, having completed a Masters of Science in Clinical Research, is in an established clinical research environment with a sponsor experienced in metabolic studies in children. Future training in nutrition and metabolic techniques are planned to further the principal investigator's education. This study will allow the principal investigator the opportunity to continue to develop clinical research skills in a mentored environment, with the expectation to expand these studies as an independent investigator in the future.